Oxygen ion conducting materials

ABSTRACT

An oxygen ion conducting ceramic oxide that has applications in industry including fuel cells, oxygen pumps, oxygen sensors, and separation membranes. The material is based on the idea that substituting a dopant into the host perovskite lattice of (La,Sr)MnO 3  that prefers a coordination number lower than 6 will induce oxygen ion vacancies to form in the lattice. Because the oxygen ion conductivity of (La,Sr)MnO 3  is low over a very large temperature range, the material exhibits a high overpotential when used. The inclusion of oxygen vacancies into the lattice by doping the material has been found to maintain the desirable properties of (La,Sr)MnO 3 , while significantly decreasing the experimentally observed overpotential.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to and is a continuation of U.S. Ser.No. 10/327,502, filed Dec. 20, 2002 now U.S. Pat. No. 6,821,498, whichis a divisional of U.S. Ser. No. 09/344,859, filed Jun. 28, 1999, nowU.S. Pat. No. 6,521,202, the entire contents of which are hereinincorporated by reference in their entirety.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.W-31-109-ENG-38 awarded by the U.S. Department of Energy. The Governmenthas certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to improved oxygen ion conductingmaterials, useful in, for example, ceramic electrolyte devices. Moreparticularly, the invention relates to a class of materials thatmaintains the excellent conductivity and catalytic properties of undopedlanthanum manganese oxide or of an A-site doped lanthanum manganiteperovskite material, while increasing oxide ion conductivity in thematerial. This increase is realized by a significantly loweroverpotential and better performance compared to other unsubstituted orA-site substituted materials.

BACKGROUND OF THE INVENTION

Ceramic electrolyte devices have a myriad of commercial uses in manydevices that require oxygen anion conductivity, ranging from powergeneration in the form of solid oxide fuel cells to oxygen pumps, oxygensensors, and air separation membranes. A ceramic electrolyte devicecomprises three separate parts: (1) a cathode that reduces elementaloxygen to oxide ions, (2) an electrolyte that transports the oxide ions,but not electrons, across a cell to an anode, and (3) the anode, wherethe oxide ions react with protons to form water. Currently, the materialmost commonly used for the cathode for these electrolyte devices is adoped lanthanum manganite, (La,A)MnO₃, a perovskite oxide, where A=Ca orSr, wherein only the lanthanum position is doped. In this class ofmaterials, where oxide examples are denoted as ABO₃, the large A-cationis typically a lanthanide, alkaline earth metal, or alkali metal cationin 7-12 coordination to oxygen. The B-cation is typically a transitionor main group metal in octahedral coordination to oxygen. The compoundLaMnO₃ has advantages in two of the three main requirements for aceramic electrolyte device material: good electrical conductivity andcatalytic activity for oxygen reduction; however, it exhibits pooroxygen ion conductivity. The poor oxygen ion conductivity problem can bepartly solved by maximizing the number of triple point boundaries in thecathode, but this solution requires careful manipulation of particlesizes and complex fabrication of the ceramic-electrolyte interface.

For LaMnO₃ based ceramic electrolyte devices, one of the mainlimitations to the technology is that overpotential in the system is toohigh, causing unnecessary energy loss and inefficiencies. Presently whenused in solid oxide fuel cells, typical LaMnO₃ based cathodes have anoverpotential of roughly 60 mV. For the cell to run more efficiently,the overpotential must be lowered. The overpotential can be lowered byincreasing the oxygen ion conductivity of the cathode.

A generally accepted method for introducing oxide ion conductivity intoceramic oxides is to substitute a lower valent element for the principlecation. For example, in zirconia, ZrO₂, vacancies can be introduced byaddition of yttria (Y₂O₃) or calcium oxide (CaO), to form, for example,Zr_(1−x)Y_(x)O_(2−x/2). In these instances, charge compensation isoxygen loss rather than reduction of zirconium cations. In othersystems, such as Li_(x)Ni_(1−x)O, an effect of substituting lithiumcations for nickel is oxidation of the nickel cations, rather thanoxygen loss. It is a balance between these two separate equilibriumsthat is a key factor in increasing utility of the LaMnO₃ system forceramic electrolyte devices. Even in doped LaMnO₃ systems, such as(La_(1−x)Sr_(x))MnO₃, a preponderance of the doping is compensated forby oxidation of the manganese cations rather than oxygen loss. A 1989study by Kuo, Anderson, and Sparlin [J. Solid State Chem. 83,52-60(1989)] on the effect of oxygen partial pressure on a chargecompensation mechanism for a doped material (La_(0.80)Sr_(0.20))MnO₃showed that manganese oxidation was favored in all cases where pO₂ wasgreater than 10⁻¹² atm at 1000° C. and 10⁻¹⁰ atm at 1200° C. In eithercase, no significant oxide ion vacancy formation was observed. Thesefindings are significant because most ceramic electrolyte cathodesoperate under conditions using a pO₂ range of 10⁻²-10⁻³ atm, well withinthe oxygen stoichiometric regime for doped LaMnO₃ materials. There isstill a need, therefore, to develop new ceramic electrolyte electrodematerials that maintain excellent conductivity and catalytic propertiesof LaMnO₃ cathode materials while increasing their intrinsic oxide ionconductivity under realistic oxygen concentrations.

It is therefore an object of the present invention to provide noveloxygen ion conducting materials and compositions.

It is another object of the present invention to provide a novellanthanum manganite material that maintains excellent conductivity andcatalytic properties, while also increasing oxide ion conductivity inthe material.

It is another object of the invention to provide a novel method fordecreasing overpotential of electrode materials.

Other objects and advantages of the invention will become apparent byreview of the detailed description of preferred embodiments.

SUMMARY OF THE INVENTION

The present invention is directed to an adjustment to a composition oflanthanum manganite electrolytes in order to enhance conductivity ofoxygen ions. Lanthanum manganites are commonly used as electrodes insolid oxide fuel cells, oxygen pumps and air separation devices. Theyare doped with strontium or calcium and have good electricalconductivity and catalytic activity but poor oxygen ion conductivity,manifested in the form of a high overpotential in fuel cells. Thisinvention is generally concerned with substitution of a lower or equalvalent element for the principal cation which will increase the oxygenion conductivity through creation of oxygen vacancies in the perovskitelattice. By increasing the oxygen ion conductivity of lanthanummanganite, energy efficiency of the system is increased, as is thematerial's potential utility at different temperatures.

The above described objects and embodiments are set forth in thefollowing description and illustrated in the drawings describedhereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the effect that substituting a tetrahedral cationinto a perovskite lattice has on surrounding cations;

FIG. 2 is a plot of measured overpotential in a cell vs. galliumconcentration in a cathode;

FIG. 3 is a plot of the change in overpotential as a function of dopantconcentration; and

FIG. 4 is a graph showing the effect of current conditioning for atypical cathode.

DETAILED DESCRIPTION

This invention is directed to oxygen ion conducting materials andcompositions, and more particularly to doped ceramic perovskites havinga general formula AA′BB′O_(x), wherein A and B are selected elements, A′and B′ are selected dopants for the elements A and B, respectively, O isoxygen, and x represents the amount of oxygen. In general, A is alanthanide or Y, and more suitably a lanthanide with preferred membersbeing elements of the group La to Gd in the periodic table. A′ is analkaline earth metal with preferred members being Mg, Ca, Sr, and Ba.Furthermore, B is a metal having multiple valances, such as manganese.B′ is further characterized as having a coordination geometry below thatfor B and preferably 5 or below. Usually, the coordination geometry forB is octahedral and the coordination geometry for B′ is tetrahedral. Thedifference in the coordination geometry for each metal contributes to aseeding of the framework with an increase in oxygen ion vacanciesthereby improving the oxygen ion conductivity and reducing theoverpotential. More particularly, B is a first row transition metal,more suitably an element of group 5 (Vanadium) to group 11 (Cu) andpreferably Mn to Ni. B′ is a late transition metal (Groups 8-12 (Fe—Zn))or a main group metal (Groups 13,14,15) with preferred members being Zn,Ga, Al and Ge.

These materials exhibit favorable oxygen ion conductivity and aretherefore useful as oxygen ion conducting components for fuel cells,particularly cathodes for solid oxide fuel cells, oxygen pumps, airseparation units, and other products. The present invention furtherincludes, in addition to these new compositions, oxygen ion conductingelectrodes and other devices, and apparatus with these oxygen ionconducting components and devices.

This invention is particularly directed to electrode materials forceramic electrolyte devices wherein the electrode materials comprise adoped lanthanum manganese oxide perovskite material of a general formulaLa_(1−x)A′_(x)Mn_(1−y)B′_(y)O_(3−δ) (0<x<1;0.01<y<0.20; B′=Al, Ga, Zn,Cu, Ni; A′=Ca, Sr). This class of materials, based on LaMnO₃, maintainsexcellent conductivity and catalytic properties, as in the undopedmaterial, while also increasing oxide ion conductivity in the material.This increase is realized by a significantly lower overpotential andbetter performance compared to other unsubstituted materials, wheredoping is restricted to A-site cation substitutions.

Significant decreases in overpotential of electrode materials can beachieved by substituting a small percentage of a metal that stronglyprefers a lower coordination number compared to the manganese cations inthe parent material. In the perovskite structure, the coordinationgeometry of the smaller (i.e., manganese) cation is octahedral. Ininstances where significant oxygen ion activity is expected, the ionsmoving through the structure do so by a percolation mechanism. In thistype of mechanism, as oxide ions enter the cathode material, they fillsurface vacancies in the perovskite framework and gradually work theirway through the material by hopping from vacancy to vacancy through thecoordination sphere of the smaller B-cation. One problem is that oxideion vacancy concentration in LaMnO₃ materials is low, even when dopingon the A-site with strontium or calcium cations. This substitutionincreases electrical conductivity of the compound, but does not increaseionic conductivity significantly.

In one form of the present invention, increasing ion conductivity hasbeen achieved by substituting a small amount of a metal cation on themanganese site that has a strong preference for a coordination numberlower than six. One example of this coordination geometry is tetrahedralor four coordination of oxygen. In this instance, adding such a cationseeds the framework with oxide ion vacancies. Because all of theoctahedra in the parent structure share common corners, adding a fourcoordinate cation would then locally break up this arrangement and causetwo of the surrounding manganese cations to also lose oxygen ions fromtheir coordination sphere. A representation of substitution of atetrahedral cation into the perovskite lattice and the effect of thesubstitution on the surrounding cations is shown in FIG. 1. Atetrahedral cation 10, in the center of FIG. 1, causes two of theadjacent octahedrally coordinated cations 12 to have a lowercoordination number. Examples of this coordination geometry for aMn(III) cation are known, i.e. Ca₂Mn₂O₅, so no unusual constraints areadded to the structure. An important consideration is that the amount ofdopant should be kept to a minimum in order to prevent clustering orsharing of vacancies by adjacent dopant cations. This is mosteffectively and easily accomplished by keeping the amount of dopant low.

The following non-limiting examples serve to further illustrateadvantages of the disclosed invention.

EXAMPLE 1

In studying the effect of the addition of a number of cations tomaterial that has a strong tendency for four coordination to LaMnO₃, theuse of gallium cations was studied as a dopant in this system. Typicalmaterials were synthesized by a glycine nitrate method and subsequentlycalcined in air at 1250° C. All samples were then screened for activityas a cathode by determining polarization behavior at 1000° C. in air ina half fuel cell arrangement. The ratio of lanthanum and strontium werevaried to establish single phase materials, as determined by powderX-ray diffraction, and the amount of gallium doped into the sample wasvaried. FIG. 2 is a plot of observed overpotential versus galliumconcentration. The three points at zero Ga dopant concentration wereobtained from the electrode compositions of La_(0.54)Sr_(0.45)MnO₃(LSM5), La_(0.59)Sr_(0.4)MnO₃ (LSM6), and La_(0.79)Sr_(0.2)MnO₃ (LSM8),respectively. Although overpotential varies with Sr content in LSMwithout Ga doping, in FIG. 2 it is evident that substitution of galliumfor manganese strongly effects the overpotential in the system. Theminima of the semicircle is approximately 5% gallium and provides thebest performance. The optimized formulation is(La_(0.55)Sr_(0.45))_(0.99)Mn_(0.95)Ga_(0.05)O_(3−δ).

Substitutions with aluminum and zinc are also believed to be able toproduce vacancy patterns similar to those observed for gallium. Nickelalso substitutes as a four coordinate cation, but has a preference forsquare planar coordination in the solid state. Divalent copper dopantsprefer five coordination in solid state oxides (square pyramidal) whiletrivalent copper (III) dopants, like nickel (II), prefer square planarconfigurations.

EXAMPLE 2

FIG. 3 shows the effect of changing the aluminum dopant concentration onthe overpotential of the Al-doped LSM cathode at a current density of250 mA/cm². The smooth curves in FIG. 3 were obtained using the samematerial for both the anode and cathode, both before and after currentconditioning at 360 mA/cm² over a week. After long term currentconditioning, anode degradation was observed, as indicated by a greatincrease in the terminal voltage between the anode and its referenceelectrode. In order to eliminate the effects of the anode degradation,similar experiments were performed using the 3% Al-doped cathode with aplatinum anode in a half-cell configuration. The results are shown intriangles in FIG. 3. As can be seen, excellent performance of thecathode is maintained when compared to standard LSM materials(approximately 60 mV) with the alternative anodes.

FIG. 4 shows a typical example of a polarization curve obtained from the5% Al-doped LSM cathode before and after current conditioning at 356mA/cm² for 5 days. The typical data shown in FIG. 4 highlights theexcellent performance and stability of the material after currentconditioning.

While preferred embodiments have been illustrated and described, itshould be understood that changes and modifications can be made thereinin accordance with one of ordinary skill in the art without departingfrom the invention in its broader aspects. Various features of theinvention are defined in the following claims.

1. A process for producing a perovskite material having increased oxygenion conductivity, comprising seeding the framework of a perovskitematerial with oxide ion vacancies by doping a B site in the perovskitelattice with a B′ dopant having a coordination geometry less than acoordination geometry of the B site, wherein the perovskite has aformula AA′BB′O_(x) in which: A is an element selected from the groupconsisting of lanthanides and Y; A′ is a dopant for A and is an alkalineearth metal; B is selected from the group consisting of Sc, V, Mn, Fe,Co, Ni, Cu and Zn; B′ is a dopant for B and is an element selected fromGroups 8-15 of the Periodic Table; and x represents the amount ofoxygen.
 2. The process of claim 1 wherein A′ is selected from the groupconsisting of Mg, Ca, Sr, and Ba.
 3. The process of claim 1 wherein Bhas multiple valences.
 4. The process of claim 1 wherein B′ is selectedfrom the group consisting of Zn, Ga, Al and Ge.
 5. A process forincreasing the efficiency of a ceramic electrolyte device, comprisingperforming a reduction of elemental oxygen to form water using anelectrode comprising a perovskite material produced by the process ofclaim
 1. 6. The process of claim 5 wherein the ceramic electrolytedevice is a solid oxide fuel cell.
 7. The process of claim 5 wherein theceramic electrolyte device is an oxygen pump or an air separation unit.8. The process of claim 5 wherein the electrode is a cathode.
 9. Anoxygen ion conducting material comprising a doped ceramic perovskitehaving a general formula AA′BB′O_(x), wherein: A is an element selectedfrom the group consisting of lanthanides and Y; A′ is a dopant for A andconsists essentially of an alkaline earth metal; B is selected from thegroup consisting of Sc, V, Mn, Fe, Co, Ni, Cu, and Zn; B′ is a dopantfor B and is an element selected from Groups 8-15 of the Periodic Tablewherein B′ has a coordination geometry less than the coordinationgeometry of B; and x represents the amount of oxygen.
 10. The materialof claim 9 wherein A′ is selected from the group consisting of Mg, Ca,Sr, and Ba.
 11. The material of claim 9 wherein B has multiple valences.12. The material of claim 9 wherein the coordination geometry of B isoctahedral and the coordination geometry of B′ is tetrahedral.
 13. Thematerial of claim 9 wherein the coordination geometry of B′ is 5 orbelow.
 14. The material of claim 9 wherein B′ is selected from the groupconsisting of Zn, Ga, Al, and Ge.